This invention relates generally to multiple color printing systems, and more particularly to the apparatus and method employed to form a full color image with a liquid toner on an intermediate surface and the transfer from that surface to a final receiving surface.
Full color copying has been achieved by using a variety of multiple color electrophotographic printing devices. Electrophotographic printing as a process employs the common feature of using a photoconductive surface that is charged to a substantially uniform potential. The photoconductive surface is then imagewise exposed to have formed thereon an electrostatic latent image corresponding to the areas of the original master that is desired to be reproduced. The electrostatic latent image of charge is developed by an appropriate developer material, such as a toner in a liquid carrier or a dry powder toner, by having the toner materials brought into contact with the electrostatic latent image. The toner particles are drawn to the charged latent image and the resultant developed or toned image is then transferred from the photoconductive surface to a final copy sheet and permanently affixed thereto by fusing.
With black and white electrophotographic copying, the previously described process is performed just once. However, with the development of multiple color electrophotographic printing, the process is repeated for each color employed and thus could require the running through of three or four or more cycles. Also, the charged photoconductive surface is exposed to a filtered light image and the resulting electrostatic latent charge is then developed with toner particles corresponding in color to the subtractive primary of the filtered light image. By way of illustration, when a red filter is employed, the electrostatic latent image is developed with cyan toner particles and the cyan toned images are transferred to the copy sheet.
This type of a process is especially employed with dry powder systems and the steps are repeated for each subsequent color employed. For example, a green filtered latent image is developed with magenta toner particles and a blue filtered light image is developed with yellow toner particles. Generally, each differently colored toner powder image is transferred sequentially to a final receiving sheet in superimposed registration so that three or more powder images are transferred sequentially to the final receiving sheet. After all are transferred, the images are fused, such as by heating or pressure, or both. A major disadvantage of these prior machines is the time involved to make the multiple passes for each color. Obviously, it is critical that the superimposed powder images be accurately and precisely aligned with one another during each part of the cycle and during transfer to the copy sheet.
Other full color printing systems utilize dry powder toner with an intermediate transfer surface or member. In most cases these are rollers, but could equally as well be belts. Again, in these types of systems the successive dry powder toner images are transferred in superimposed registration from the photoconductive drum to the intermediate roller and then to the final receiving surface. Multiple photoconductor drums could also be used.
Liquid toner processes and copiers have become more popular. The liquid developer material includes a liquid carrier having toner particles dispersed therein and the liquid toner is brought into contact with the electrostatic latent image, allowing the toner particles to be deposited thereon in imagewise configuration. Once the liquid carrier toner particles have been deposited on the photoconductive surface in imagewise configuration, the image is transferred to a copy sheet either directly, or through an intermediate transfer surface.
One disadvantage of using liquid toner developers is that the copy sheet is wet with both the toner particles and the liquid carrier. It is therefore necessary to remove the liquid carrier from the copy sheet. Various drying systems have been devised to do this, either prior or subsequent to the fusing step, by vaporizing the liquid carrier. The use of an intermediate transfer surface, such as a web, belt or roller, facilitates the removal of the liquid carrier. However, this intermediate transfer surface also requires an additional transfer, which affects the quality of the transferred image. Generally, liquid images tend to smear and intermingle with one another, distorting and blurring the resulting multiple color copy. Liquid images have also been known to transfer back to the photoconductive surface from the intermediate or final surface.
A number of representative patents dealing with an intermediate transfer belt for both dry powder and liquid toner developer materials do this.
U.S. Pat. No. 2,990,278 issued Jun. 27, 1961 to Carlson discloses a method and apparatus for transferring and fusing a dry powder toner from a xerographic drum via an intermediate transfer belt to a final copy sheet.
U.S. Pat. No. 3,893,761 issued Jul. 8, 1975 to Buchan et al. discloses an apparatus with an intermediate transfer belt made from silicone rubber that transfers the toned image from a first support material via the use of pressure to a second support material, such as a final copy sheet, by the use of heat and/or pressure. A radiant heater is employed to heat the dry powder toner on the silicone belt prior to transfer.
U.S. Pat. No. 3,923,392 issued Dec. 2, 1975 to Buchan et al. discloses an apparatus with an intermediate transfer belt made from silicone elastomer which pressure transfers the dry powder image from the photoreceptor to the final transfer fusing station. A radiant heater is utilized in the intermediate transfer.
U.S. Pat. No. 4,095,866 issued Jun. 20, 1978 to Koeleman et al. discloses a photoreceptor belt, an intermediate transfer belt, and a final transfer and fusing apparatus. The first transfer station employs pressure on the intermediate transfer belt which is made of silicone rubber and heated. The image is transferred to the final receiving surface by pressure and contact.
U.S. Pat. No. 4,348,098 issued Sep. 7, 1982 to Koizumi discloses an apparatus and process alleged to be equally adaptable to a dry powder toner or liquid toner in which an intermediate transfer drum is electrically biased by corona charging to transfer the toned image thereto. Toned images are then transferred and fused to a final receiving sheet by pressure rolls.
U.S. Pat. No. 4,445,820 issued Jun. 12, 1984 to Suzuki discloses an intermediate transfer belt made from silicone or RTV rubber which receives a toned image from a photoconductive drum through light pressure. The toned image is then heated to fusion temperature on the belt and then the final receiving surface is pressed against the belt to effect transfer of the toned image.
U.S. Pat. No. 4,542,978 issued Sep. 24, 1985 to Tarumui et al. discloses a high speed transfer device that uses a photoconductor and an intermediate belt wherein transfer is accomplished by light contact between the belt and the photoconductor. The intermediate belt wraps about a large diameter heating roller and the toner image is subsequently transferred to the final receiving surface through pressure between the heating roll and the second roll.
U.S. Pat. No. 4,690,539 issued Sep. 1, 1987 to Radulski et al. discloses apparatus in which liquid toned images are transferred from a photoconductive member to a final receiving sheet via an intermediate web. The liquid carrier is removed from the intermediate web by means of a vacuum that also secures the toner particles to the intermediate web. The full color image is transferred from the intermediate transfer surface to the final receiving transfer surface at a transfer station by a corona transfer by spraying charged ions on the backside of the final receiving surface.
U.S. Pat. No. 4,708,460 issued Nov. 24, 1987 to Langdon discloses an apparatus in which the liquid image is transferred from a photoconductive member to an intermediate member positioned closely adjacent thereto but not in contact therewith. The liquid image is simultaneously transferred and fused to the final receiving sheet from the intermediate transfer member by a contact transfer.
U.S. Pat. No. 4,935,788 issued Jun. 19, 1990 to Fantuzzo et al. discloses a multiple color printing system for a liquid toner utilizing a plurality of developing stations about the photoconductor. The different color liquid images are transferred in superimposed registration to the intermediate member to form a multiple color liquid image thereon and thereafter the multiple color liquid image is transferred to the final receiving sheet and fused thereto.
In all of these prior art references, the quality of the transferred image has not been preserved, as previously explained. These problems are solved in the design of the present invention utilizing a conductive intermediate surface that is positioned adjacent to the photoconductor and a second intermediate transfer adhesive surface that receives a dried toned image from the conductive intermediate transfer surface and transfers it to the final receiving surface via pressure and heat.